An atmospheric diffuser is a successful piece of equipment for treating paper pulp and the like. Although such structures can be used only for thickening, typically diffusers are used for washing, bleaching, or otherwise treating pulp, with a treatment liquid. The diffusers are often provided with multiple stages, having a number of screen assemblies. Typical multiple stage diffusers are illustrated in U.S. Pat. No(s). 3,815,386 and 4,172,037, the disclosures of which are incorporated by reference herein.
In conventional atmospheric diffusers, normally the pulp flows upwardly in the diffuser vessel, flowing past the bottom and then past the top of screens extending downwardly from an arm which receives liquid which passes through the screens, for ultimate withdrawal from the vessel. In tests on the efficiency of diffusers, it was surprisingly found that the lower screens in each stage--the screens that extend downwardly from the mounting arm, when the pulp is flowing upwardly in the vessel--had significantly less efficiency than the upper screens of each stage. After substantial investigation and analysis, it was determined that the source of this problem of relatively poor screen efficiency was contamination of the cleanest portion of the pulp as it flowed upwardly with backflush liquid that was dirtier than that just withdrawn from the pulp at the upper end of the screen.
In order to solve the above identified problem, according to the present invention, the pathway that the liquid being withdrawn from the pulp must take as it passes through the screens, is down toward the bottom of the screen. Thus, when backflushing action occurs by rapid downward movement of the screen, the backflushing liquid at the top of the screen will be relatively clean liquid which has just passed through the screen at that point, rather than much dirtier liquid in conventional atmospheric diffusers. The pulp is preferably caused to take this path by providing barriers--such as tubes or channels--within the screens with passages adjacent the second ends of the screens. In order to make the backflushing action as uniform as possible, a baffle is mounted in front of the passage--between the passage and the screen--to redirect the backflushing liquid flowing through the passage.
According to one aspect of the present invention, an atmospheric diffuser for treating cellulosic fibrous material pulp is provided. The diffuser comprises: a generally upright vessel; a plurality of arms mounted within the vessel for up and down movement; a plurality of annular screen assemblies mounted on the arms, each assembly including at least one screen; conduit means for transporting liquid passing into the screen assemblies from pulp in the vessel, into the arms and out of the vessel; at least one screen of each screen assembly having an outer face and an inner face, the outer face first encountering the pulp as it moves in its direction of movement, with the arms operatively connected to the screen face at the downstream end thereof in the direction of pulp movement; means for moving the arms up and down in movement coincident with pulp flow in one direction and to cause backflushing of liquid through the screens in the other direction; and means for directing backflushing liquid from the inner face of the screen back through the screen so that the backflushed liquid passing through the screen adjacent the downstream end thereof has substantially the same properties as liquid just withdrawn through the screen at that point. The means for directing the backflushing liquid preferably comprise barrier means with passages in the barrier means adjacent the second end of the screens. Baffle means are mounted between the passages and the screen innerface for redirecting the backflushing fluid.
The barrier means preferably comprise a plurality of tubes and are connected by webs, or a plurality of channels that connected by webs. Preferably, each screen assembly includes two concentric screens with the barriers disposed within the annular space between the screens, in which case the barrier tubes or channels may each have two passages therein--one facing each screen--or alternate tubes or channels can have a passage facing one way, with the next tube or channel passage facing the other way.
According to another aspect of the present invention, a method of treating cellulosic fibrous material pulp in an upright vessel is provided. The method comprises the steps of: (a) causing the pulp to flow in a first vertical direction in the vessel, the pulp when flowing in the first direction initially encountering the second end of the screen, and the flowing to and past the first end thereof, some liquid in the pulp passing through the screen; (b) withdrawing liquid flowing through the screens from the vessel; (c) periodically causing a flow of backflushing liquid back through the screen into the pulp; and (d) controlling the backflushing liquid so that the liquid backflushing the screen at the first end thereof is liquid that passed through the screen substantially at the first end of the screen. The method also preferably comprises the step (e) of introducing treatment liquid into the pulp adjacent the annular screen. Step (d) is practiced by defining a path requiring flow of pulp passing through the first end of the screen to the second end of the screen. The first vertical direction--the direction of flow--of the pulp preferably is upwardly, and the screen is moved slowly upwardly with the pulp when treatment liquid is passing through the screen, and is moved quickly downwardly to effect backflushing.
According to another aspect of the present invention, a screen assembly per se is provided. The screen assembly comprises: first and second annular bars; at least one annular screen; the screen connected at a first end thereof to the first bar, and at the second end thereof to the second bar; means defining a plurality of spaced through extending openings in the first bar; a barrier extending from the first bar substantially to the second bar and spaced from the screen, the barrier separating the screen from the plurality of openings; and means defining a plurality of passages in the barrier adjacent the second end of the screen so that liquid passing through the screen must flow through the passage to enter the openings. Preferably two concentric screens are provided with the barrier between them, and the barrier comprises a plurality of tubes separated by webs, or a plurality of channels separated by webs.
It is a primary object of the present invention to provide for the more efficient treatment of pulp or the like in atmospheric diffusers or the like. This and other objects of the invention will become clear from an inspection of a detailed description of the invention, and from the appended claims.